In typical applications, semiconductor devices are operated by, and are used to control, the flow of electric current within specific circuits to accomplish particular tasks. Accordingly, because of their high conductivity, the most useful and convenient materials for carrying current from device to device are metals.
In order to connect devices into useful circuits, appropriate contacts must be made between semiconductor devices and the metals--e.g. printed circuits, wires, or any other appropriate metal elements--used to carry current. Often, the most appropriate contacts are also formed of metal. Such metal contacts should interfere either minimally or preferably not at all with the operation of either the device or the current carrying metal. The contact must also be physically and chemically compatible with the semiconductor material.
In this regard, the term "ohmic contact" is used to define such an appropriate metal-semiconductor contact. Specifically, an ohmic contact can be defined as a metal-semiconductor contact that has a negligible contact resistance relative to the bulk or spreading resistance of the semiconductor, Sze, Physics of Semiconductor Devices, Second Edition, 1981, page 304. As further stated therein, an appropriate ohmic contact will not significantly change the performance of the device to which it is attached, and it can supply any required current with a voltage drop that is appropriately small compared with the drop across the active region of the device.
An ohmic contact can also be qualified using other characteristics of both a metal and a semiconductor. In most cases, and in addition to the other necessary physical and chemical characteristics, in order to act as an ohmic contact to a particular p-type semiconductor, the metal must have a work function greater than the work function of the semiconductor. Ideally, the work function of the metal should be greater than the electron affinity and the bandgap of the semiconductor.
As known to those familiar with this art and this terminology, the work function is defined in terms of the Fermi energy of the material. In turn, the Fermi energy of a material is the effective average energy of the electrons in thermal equilibrium with the surroundings of the material. Alternatively, the Fermi energy can be defined as the energy at which half the available states are on the average actually populated with electrons at thermal equilibrium. The work function of the material is the energy required to remove an electron having the Fermi energy from the material to an infinite distance away from the material.
Summarized somewhat differently, if the work function of the metal is greater than that of the p-type semiconductor, it may qualify as an ohmic contact material to that p-type semiconductor. If, however, the work function of the semiconductor is greater than that of the metal, it will be difficult or impossible to establish ohmic behavior and rectifying behavior may be demonstrated instead.
One material for which great semiconductor potential has long been recognized is silicon carbide (SiC). Silicon carbide has well known advantageous semiconductor characteristics: a wide bandgap, a high thermal conductivity, a high melting point, a high electric field breakdown strength, a low dielectric constant, and a high saturated electron drift velocity. Taken together, these qualities potentially give electronic devices formed from silicon carbide the capability to operate at higher temperatures, higher device densities, higher speeds, higher power levels, and even under higher radiation densities, as compared to other semiconductor materials. Accordingly, attempts to produce appropriate devices from silicon carbide, as well as attempts to produce device quality silicon carbide itself, have been of interest to scientists and engineers for several decades. As stated above, one aspect of device manufacture in any semiconductor material, and specifically including silicon carbide, is the ability to produce appropriate ohmic contacts.
When used as a semiconductor material, particularly p-type, silicon carbide presents special challenges with respect to ohmic contacts because of its relatively large bandgap which results in a relatively large work function as well. For example, in the ideal case, disregarding any effects of band bending caused by Fermi level pinning, p-type alpha (6H) silicon carbide has a work function ranging from about 5.7 to 7.2 electron volts (eV) depending upon the carrier concentration. For those well skilled in the art, the work function is defined as the sum of the electron affinity plus the energy level between the conduction band and the Fermi level. As a result, finding metals with a work function greater than that of silicon carbide has to date proved difficult and troublesome. The same problems hold true for other wide bandgap semiconductor materials such as zinc selenide (ZnSe), gallium nitride (GaN), diamond, boron nitride (BN), gallium phosphide (GaP), and aluminum nitride (AlN).
As a result, ohmic contacts to p-type silicon carbide are generally formed of alloys such as aluminum-titanium and aluminum-silicon which have to be annealed at relatively high temperatures (e.g. 900.degree. C.) in order to form an appropriate ohmic contact. As known to those familiar with such devices, the repeated exposure of a device to such high temperatures will eventually generally change its character. Furthermore, the presence of aluminum or aluminum alloys on silicon carbide-based semiconductor devices limits the later treatment or processing of such devices because of the limitations of the physical and chemical properties of the alloys and the alloying metals, particularly their somewhat lower melting points.
Platinum (Pt), because of its characteristics as a noble metal, is a desirable candidate for ohmic contacts to all sorts of semiconductor materials. Unfortunately, and with respect to the parameters discussed earlier, the work function of platinum is 5.65 eV; i.e. generally about 0.6 eV less than that of p-type 6H silicon carbide. Thus, approached from a bandgap theory, platinum should not provide an ohmic contact to silicon carbide.
Therefore, the need exists for ohmic contacts to p-type silicon carbide that can use metals more preferable than those presently most commonly incorporated. There is a further need for a contact that does not require high temperature annealing in order to perform as an ohmic contact, and for an ohmic contact which maintains its ohmic character after any necessary or desired subsequent annealing of the device or silicon carbide.